Burners of the type here considered generally have a central pipe through which oil (or sometimes gas) is fed to a nozzle where it is ignited by electronic means such as a spark plug. This pipe is coaxially surrounded by several tubes forming channels for the passage of air and fuel which intermix in the region of the nozzle so as to be fired by its flame. The air flow may be set in rotation by passing through a twist-blade ring or impeller, e.g. as taught in U.S. Pat. No. 4,333,405, for a thorough intermingling with the fluid which is separately conveyed to the nozzle area. While this fluid may also be oil, its high cost and occasional scarcity are conducive to the use of cheaper combustibles such as coal dust; this is also described in the above-identified U.S. patent.
Hard-coal or anthracite powder has been used, but more commonly brown-coal nozzles are employed in areas where this cheap fuel is readily available. Usually such coal nozzles are wholly separate from the oil nozzles which must be employed to maintain proper combustion. In any case it is normally considered impossible to use better than a 1:1 or 1:2 mixture by mass of coal to air, that is the nozzles cannot use less than 1 kg of air to fluidize and effectively burn more than 1 kg or 2 kg of coal so that a high proportion of expensive oil must be used even when conditions permit coal burning.
A brown-coal fuel, known as TBK 10, is a by-product of the briquetting of brown coal or lignite. It has a particle size with at least 43% by weight of 90-micron and 16% by weight of 200-micron particles, and a heat value of 21.4 MJ/kg. In spite of the attractiveness of this fuel, it has not been successfully used in a power-plant burner of the type discussed above.